538 Chapter 16
expiration ( table 16.1 ), causing the lungs to stick to the chest and
thereby produce changes in lung volume as the thoracic volume
changes. During inspiration, it is the transpulmonary pressure
that causes the lungs to expand as the thoracic volume expands.Boyle’s Law
Changes in intrapulmonary pressure occur as a result of changes in
lung volume. This follows from Boyle’s law, which states that the
pressure of a given quantity of gas is inversely proportional to its
volume. An increase in lung volume during inspiration decreases
intrapulmonary pressure to subatmospheric levels; air therefore
goes in. A decrease in lung volume, conversely, raises the intrapul-
monary pressure above that of the atmosphere, expelling air from
the lungs. These changes in lung volume occur as a consequence
of changes in thoracic volume, as will be described in section 16.3.Physical Properties of the Lungs
In order for inspiration to occur, the lungs must be able to expand
when stretched; they must have high compliance. For expiration
to occur, the lungs must get smaller when this tension is released:
they must have elasticity. The tendency to get smaller is also
aided by surface tension forces within the alveoli.Compliance
The lungs are very distensible (stretchable)—they are, in fact,
about a hundred times more distensible than a toy balloon.
Another term for distensibility is compliance, which here refers
to the ease with which the lungs can expand under pressure.
Lung compliance can be defined as the change in lung volume
per change in transpulmonary pressure, expressed symbolically
as Δ V /Δ P. A given transpulmonary pressure, in other words,
will cause greater or lesser expansion, depending on the com-
pliance of the lungs. Although it is controversial, it may be that
the alveolar ducts and sacs, rather than the individual alveoli,
are the major structures that expand in size during inspiration,
while the inflation of alveoli is due primarily to alveolar recruit-
ment (the filling with of more alveoli with air).
The compliance of the lungs is reduced by factors that pro-
duce a resistance to distension. If the lungs were filled with
concrete (as an extreme example), a given transpulmonary
pressure would produce no increase in lung volume and no air
would enter; the compliance would be zero. The infiltration of
lung tissue with connective tissue proteins, a condition called
pulmonary fibrosis, similarly decreases lung compliance.Elasticity
The term elasticity refers to the tendency of a structure to
return to its initial size after being distended. Because of their
high content of elastin proteins, the lungs are very elastic and
resist distension. The lungs are normally stuck to the chest
wall, so they are always in a state of elastic tension. This ten-
sion increases during inspiration when the lungs are stretched
and is reduced by elastic recoil during expiration.contains only a thin layer of fluid, secreted by the parietal
pleura. This fluid is like the interstitial fluid in other organs;
it is formed as a filtrate from blood capillaries in the parietal
pleura, and it is drained into lymphatic capillaries. The major
function of the liquid in the intrapleural space is to serve as a
lubricant so that the lungs can slide relative to the chest during
breathing. Since the lungs normally are stuck to the thoracic
wall, for reasons described shortly, they expand and contract
with the thoracic wall during breathing. The intrapleural space
is thus more a potential space than a real one; it becomes real
only if the lungs collapse.
Air enters the lungs during inspiration because the atmo-
spheric pressure is greater than the intrapulmonary, or
intra-alveolar, pressure. Because the atmospheric pressure
does not usually change, the intrapulmonary pressure must
fall below atmospheric pressure to cause inspiration. A pres-
sure below that of the atmosphere is called a subatmospheric
pressure, or negative pressure. During quiet inspiration,
for example, the intrapulmonary pressure may decrease to
3 mmHg below the pressure of the atmosphere. This subat-
mospheric pressure is shown as 2 3 mmHg. Expiration, con-
versely, occurs when the intrapulmonary pressure is greater
than the atmospheric pressure. During quiet expiration, for
example, the intrapulmonary pressure may rise to at least
1 3 mmHg over the atmospheric pressure (see fig. 16.14 ).
Because of the elastic tension of the lungs (discussed
shortly) and the thoracic wall on each other, the lungs pull in one
direction (they “try” to collapse) while the thoracic wall pulls
in the opposite direction (it “tries” to expand). The opposing
elastic recoil of the lungs and the chest wall produces a subat-
mospheric pressure in the intrapleural space between these two
structures. This pressure is called the intrapleural pressure.
As indicated in table 16.1 , the intrapleural pressure is lower
(more negative) during inspiration because of the expansion of
the thoracic cavity than it is during expiration. However, the
intrapleural pressure is normally lower than the intrapulmonary
pressure during both inspiration and expiration ( table 16.1 ).
There is thus a pressure difference across the wall of the
lung—called the transpulmonary (or transmural ) pressure —
which is the difference between the intrapulmonary pressure
and the intrapleural pressure. Because the pressure within the
lungs (intrapulmonary pressure) is greater than that outside the
lungs (intrapleural pressure), the difference in pressure (trans-
pulmonary pressure) keeps the lungs against the chest wall. The
transpulmonary pressure is positive during both inspiration and
Table 16.1 | Pressure Changes in Normal,
Quiet Breathing
Inspiration Expiration
Intrapulmonary pressure (mmHg) 23 13
Intrapleural pressure (mmHg) 26 23
Transpulmonary pressure (mmHg)^1316Note: Pressures indicate mmHg below or above atmospheric pressure.